Method of erecting a wind turbine

ABSTRACT

A method of erecting a wind turbine on a wind turbine site, the wind turbine comprising a turbine tower and a nacelle. The method comprises the following steps: providing a plurality of tower sections being arrangeable upon each other in a vertical orientation in a tower structure to form the turbine tower; providing at least one damper unit configured to dampen oscillations of the turbine tower; attaching a damper unit to one of the plurality of the tower sections on an outside thereof, and subsequently arranging the tower section in the tower structure, and arranging the nacelle on top of the tower structure while the damper unit is attached to a tower section in the tower structure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/254,343, filed Dec. 21, 2020 (pending), which is a U.S. NationalPhase Application of PCT Application No. PCT/DK2019/050207, filed Jun.26, 2019 (expired), which claimed the priority of Danish Application No.2018 70449, filed Jun. 29, 2018, the disclosures of which areincorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The present invention relates to a method of erecting a wind turbine byuse of a damper unit.

BACKGROUND OF THE INVENTION

Modern wind turbines are used for generating electrical energy for apower grid. To this end, a set of wind turbine blades coupled to a rotorare directed into the incoming wind, and the energy of the wind isextracted by the wind turbine blades and causes the rotor to rotate,thereby converting the energy of the wind into mechanical energy. Therotor is connected to a generator, either directly or via a geararrangement, and thereby the mechanical energy of the rotating rotor isconverted into electrical energy. The electrical energy is supplied to apower grid, via suitable components.

Over the years, the size and dimensions of the wind turbines havegenerally increased in order to increase the amount of energy extractedfrom the wind and the annual power output. With the significantly longerblades and larger generators follow correspondingly higher wind turbinetowers, larger and significantly heavier nacelles and larger bearingsetc. The tower and foundation thereby naturally need to be dimensionedto carry the larger loads and vibrations caused by the wind acting onthe larger and heavier system.

Consequently, a wind turbine tower often comprises a plurality of towersections. When erecting the wind turbine, the plurality of towersections are installed on top of each other, and a nacelle is installedon top of the upper most tower section.

During installation of the tower sections and the nacelle,Vortex-induced vibrations may occur. This is due to the phenomena knownas vortex shedding that is a particular problem with high slender towersof a height from approximately 80 meters and upwards.

Vortex shedding occurs due to instability of the flow around an object,such as a wind turbine tower. Low-pressure vortices are created on thedownstream side of the tower and intermittently detach from either sideof the tower. The tower will tend to move towards the low pressure,i.e., an alternating force is applied to the tower. The frequency bywhich the force alternates from side to side depends on the diameter ofthe tower and the wind speed. At a certain wind speed the frequency ofthe alternating force coincides with the natural frequency of the windturbine tower, i.e., the tower frequency of the wind turbine tower. Thiswind speed is known as the critical wind speed. At this wind speed thetower will start to oscillate.

The amplitudes of the oscillations at the critical wind speeds depend onthe structural damping of the wind turbine tower. If no additionaldamping is added to the wind turbine tower the oscillations can resultin severe deflections of the wind turbine tower.

The effects of vortex shedding are especially pronounced duringconstruction of the tower until the nacelle has been installed, as thischanges the natural frequency of the system. Should the tower start tooscillate at some point during construction of the tower, it isnecessary to stop further construction and wait until the wind speed isbelow the critical wind speed again. In such a situation it isimpossible to apply mitigating measures to stop the oscillations.Personnel safety may be compromised and structural damage to the towermay occur. Therefore, it is required to have a weather window with windspeeds below the critical wind speed during construction of the toweruntil the nacelle has been installed.

This may prolong the erection time of the wind turbine.

DESCRIPTION OF THE INVENTION

It is an object of embodiments of the invention to provide an improvedmethod of erecting a wind turbine.

It is a further object of embodiments of the invention to provide amethod of reducing the risk of vortex induced vibrations during erectionof a wind turbine tower.

According to a first aspect, the invention provides a method of erectinga wind turbine on a wind turbine site, the wind turbine comprising aturbine tower and a nacelle, the method comprising the following steps:

-   -   providing a plurality of tower sections being arrangeable upon        each other in a vertical orientation in a tower structure to        form the turbine tower,    -   providing at least two damper units configured to dampen        oscillations of the turbine tower,    -   attaching one of the at least two damper units to one of the        plurality of the tower sections on an outside thereof, and        subsequently arranging said tower section in the tower        structure,    -   attaching a subsequent one of the at least two damper units to        another one of the plurality of the tower sections on an outside        thereof, and subsequently arranging said tower section in the        tower structure, while the previous one of the at least two        damper units is still attached to the previous one of the        plurality of tower sections.

It is herewith achieved that oscillations of the tower structure can bedampened at all times during the part of the erection where the towerstructure has a height where vortex induced vibrations can occur. Thus,the critical wind speed where vortex induced oscillations occur ishigher and thereby allows for erecting towers in higher wind speeds.Therefore the allowable weather window for installing towers can beexpanded. In the context of the present invention, the term “weatherwindow” should be understood as a time period where the weather does nothinder erection of the turbine tower.

Since the damper is arranged on the outside of the tower section, it ispossible to work inside the tower section during assembly of the towersections as the damper does not take up space inside the tower, andsince the tower section is arranged in the tower structure after thedamper is attached on the outside, the damper can operate to dampenoscillations already during the assembly of the tower. Moreover, it iseasier to mount a damper on the ground as compared to mounting it inhigh altitude.

After erecting the wind turbine, the wind turbine comprises an elongatedturbine tower which is formed by a plurality of tower section arrangedupon each other in a vertical orientation and a nacelle. The towersections may each be of the same height. However, at least some of thetower sections may be different height.

When erecting the wind turbine the lowermost turbine section may bearranged on a foundation.

When arranging a tower section upon another tower section, the twoneighbouring tower sections may be attached to each other e.g., byfixing a top tower flange of the lower one of the tower sections to abottom tower flange of the upper one of the tower sections.

When the uppermost tower section has been arranged, a nacelle may bearranged on top of the turbine tower.

The method comprises a step of providing a plurality of tower sectionsbeing arrangeable upon each other in a substantially vertical towerstructure to form the turbine tower. The plurality of tower sections maybe arranged at the site where the wind turbine is to be erected. Thetower structure may be erected on a pre-cast foundation for the windturbine tower. The tower structure may comprise one tower section ormore tower sections arranged upon each other. Thus, in the context ofthe present invention, the term “tower structure” should be understoodas the part of the turbine tower which has been erected at any timeduring the erection of the wind turbine. Consequently, the height of thetower structure increases during erection of the wind turbine as moretower sections are installed, so that the tower structure at a laterstage may comprise a plurality of tower sections.

The damper units which are configured to reduce oscillations of theturbine tower may each comprise an active damper or a passive damper.Advantageously the damper may be a passive tuned mass damper, where thedamping frequency is manually or automatically adjustable to cater forthe variations of the natural frequency of the tower especially duringthe later stages of the erection of the wind turbine tower.

In one embodiment, the damper units may comprise a damper unit structureadapted for attachment to the tower structure, a pendulum structure, asuspension arrangement for suspending the pendulum structure from thedamper unit structure such that the pendulum structure is allowed todisplace from a neutral position for the pendulum structure, thesuspension arrangement comprising one or more wires for suspending thependulum structure, a sensor adapted for measuring movements of thetower structure, and tuning means configured for adjusting the naturalfrequency of the suspended pendulum structure in response to measuredmovements of the tower structure.

The method comprises a step of attaching one of the damper units to oneof the plurality of tower sections on an outside thereof, which towersection is a tower section positioned on the site and being configuredto be arranged in the tower structure to thereby form part of theturbine tower.

Subsequent to the step of attaching the damper unit to the towersection, the tower section is arranged in the tower structure.Consequently, a tower section is arranged in the tower structure with adamper unit attached hereto.

As mentioned above, the tower structure is erected upon a preparedfoundation for the wind turbine tower and may comprise one tower sectionor more tower sections arranged upon each other. Thus, the abovementioned tower section with the damper unit attached may be arranged onthe foundation and attached hereto when arranging the tower section inthe tower structure. However, as would be more common, the tower sectionfirst having a damper unit attached during the erection may be arrangedon top of a partly finished tower structure comprising one or more towersections having previously been arranged in the tower structure andattached thereto.

The method further comprises a step of attaching a subsequent damperunit to another tower section on an outside thereof, and a step ofarranging this tower section in the tower structure. As this damper unitis attached to a subsequent tower section which is arranged in the towerstructure while the previous damper unit is still attached to theprevious tower section, it is achieved that at least one damper unit canbe attached to the tower structure while erecting the turbine tower, andthereby ensure that dampening of oscillations is possible during theentire erection process. Moreover, it may be achieved that at least onedamper unit is attached to the tower structure at all times during theremaining part of the erection process after arranging a tower sectionin the tower structure for the first time before the tower section has aheight where vortex induced vibrations may become critical.

The subsequent tower section may be arranged on top of the previoustower section whereby these tower sections are two neighbouring towersections. It should however, be understood, that another tower section,such as a tower section without a damper unit attached hereto can bearranged between these tower sections, such as on top of the previoustower section prior to the step of arranging the subsequent towersection with the subsequent damper unit in the tower structure.

In a further embodiment, a plurality of tower sections without a damperunit attached hereto are arranged on top of the previous tower sectionprior to the step of arranging the subsequent tower section with thesubsequent damper unit in the tower structure.

The nacelle may be arranged on top of the tower structure while at leastone of the two damper units is attached to a tower section in the towerstructure. Consequently, vortex induced vibrations may be prevented orat least reduced throughout the entire erection procedure untilcompletion of the nacelle mounting or throughout the remaining erectionprocedure after mounting a tower section with a damper unit for thefirst time. Subsequent to the attachment of the nacelle, the at leastone damper unit may be removed, as the nacelle together with the turbinetower may be designed with a natural frequency that does not coincidewith the frequency where vortex induced vibrations occur, whereby aseparate damper unit may be omitted.

The method may comprise a further step of attaching a hoisting system toat least one of the damper units and releasing said damper unit from thetower section to which it is attached in the tower structure andlowering said damper unit to the site.

After lowering the damper unit to the ground at the site, the damperunit may be attached to a subsequent tower section which may e.g., belocated at the ground or on transportation means, such as a vehicle, atthe site, where the wind turbine is erected.

The subsequent tower section may be arranged in the tower structure ontop of a tower section to which a damper unit is attached or on top ofanother tower section, i.e., a tower section without a damper unitattached.

The hoisting system may as an example form part of a hoist, a winch, ora crane structure to thereby enable lifting of a tower structure and/ora damper unit.

The step of arranging one of the tower sections in the tower structuremay comprise a step of lifting the hoisting system and said towersection, arranging the tower section in the tower structure, detachingthe tower section from the hoisting system, attaching the hoistingsystem to one of the damper units attached to another tower section inthe tower structure, and lowering the hoisting system and said damperunit.

Thus, the hoisting system may be attached to a damper unit which isattached to a tower section in the tower structure, and after releaseherefrom lowered to another tower section on site and attached hereto.By performing this step subsequent to a step of attaching another damperunit to another tower section and arranging said tower section in thetower structure, it may be achieved that a damper unit is still attachedto the tower structure, when the previous damper unit is released from aprevious tower section in the tower structure.

The step of arranging a subsequent tower section in the tower structureand releasing a previous damper unit may be carried out by steps ofattaching a hoisting system to the subsequent tower section to therebyenable lifting hereof to arrange it in the tower structure, subsequentlylifting the hoisting system and the subsequent tower section, arrangingthe subsequent tower section in the tower structure, detaching thesubsequent tower section from the hoisting system, attaching thehoisting system to the previous damper unit, and lowering the hoistingsystem and the previous damper unit. Consequently, the step of arrangingthe subsequent tower second in the tower structure and the step oflowering the previous damping unit may be carried out without anintermediate step of lowering the hoisting structure. Thus, the numberof lifting operations is optimized.

In one embodiment, the step of attaching the hoisting system to one ofthe damper units in the tower structure is carried out before the stepof detaching the tower section from the hoisting system. The step ofattaching the hoisting system to the damper unit attached to a towersection on an outside thereof in the tower structure may be performed byworkers who are ready to attach the next tower section to the towersection which is presently the uppermost tower section in the towerstructure. The workers may stand on a platform inside the tower sectionin the tower structure from which position they can attach the hoistingsystem to the damper unit attached to the tower section in the towerstructure before the next tower section is brought into alignment withthe tower structure and attached to the uppermost tower section.

It should however, be understood that the hoisting system in analternative embodiment may be attached to the damper unit by robot meanswhich may e.g., be controlled by a controller for controlling thehoisting system.

The hoisting system may comprise a tower section lifting sling and adamper unit lifting sling. Each sling may comprise attachments meansconfigured for attachment of a tower section and a damper unit,respectively, e.g., by engagement with a corresponding attachmentfeature on the tower section and the damper unit. The damper unitlifting sling have a length exceeding a height of a lifted towersection, whereby the step of attaching the hoisting system to a damperunit may be carried out while the hoisting system is attached to anupper end of a tower section being arranged on top of a tower sectionalready in the tower structure to which tower section, the damper unitis attached.

In one embodiment, the step of attaching one of the at least two damperunits to one of the plurality of the tower sections on an outsidethereof for the first time, and subsequently arranging said towersection in the tower structure may be carried out when a predeterminednumber of tower sections has been arranged in the tower structure. As analternative, the step may be carried out when the tower structure hasreached a predetermined height. The predetermined number of towersections and/or the predetermined height may be specified based on anassessment of and/or a calculation of the risk of occurrence ofoscillations of the tower structure. The assessment/calculation may asan example include weather data for the specific site and data relatingto the layout of the tower sections, such as height, diameter, weight,etc.

It should be understood, that the steps of attaching one of the damperunits to a tower section and subsequently arranging the tower sectionwith the damper unit in the tower structure may be repeated a pluralityof times until all tower sections are arranged in the tower structureand the turbine tower is erected. It should be understood, that sometower sections may be arranged with a damper unit attached to an outsidethereof, whereas other tower sections may be arranged without a damperunit attached.

The at least two damper units may be alternatingly attached to a towersection, whereby a damper unit may be arranged at the tower structurethe entire time after arranging a tower section with a damper unit inthe tower structure for the first time.

The step of releasing one of the damper units from a tower section inthe tower structure may comprise a step of determining tension in thehoisting system, and based on the determined tension, to select a statewhere said damper unit can be released from the tower section. Bysensing tension in the hoisting system, it may be possible to detectwhether the damping unit is attached to the hoisting system and fullysupporting the weight of it or not. Consequently, by monitoring tensionit may be possible to determine whether the hoisting system has grabbedthe damping unit or not. If the determined tension is below apredetermined threshold value, the damping unit may not be released. Oncontrary, if the determined tension is above a predetermined thresholdvalue, the damping unit may be released, as the determined tension mayprove, that the hoisting system has grabbed the damping unit which mayconsequently be released from the tower section.

A previous step of lifting the hoisting system may facilitatedetermination of tension in the hoisting system. Thus, the step oflowering the hoisting system and at least one of the damper units may besubsequent to a step of lifting the hoisting system and the damper unit.

The step of lowering one of the damper units may comprise a step ofrotating said damper unit approximately 90 degrees before attaching thedamper unit to the a subsequent one of the plurality of tower sections.Dependent on the location and/or orientation of the tower sections onground or on transportation means or on other means on the wind turbinesite, it may be necessary to attach the damping unit at differingpositions on the outside of the different wind turbine sections. As anexample, a damper unit may, in one embodiment, be attached in a 12o'clock position of a horizontal tower section on an outside thereof,whereas the same damper unit may in another embodiment be attached in a3 or 9 o'clock position of a horizontal tower section on an outsidethereof.

If a damper unit is not to be attached to another tower second, whenreleased and lowered from a tower section, it may be an advantage if thedamper unit in question can be parked at the site until the damper unitis to be used again. Consequently, the step of lowering a damper unitmay comprise a further step of parking said damper unit on the sitebefore attaching said damper unit to a subsequent tower section. Thestep of parking the damper unit may form part of the step of rotationthe damper unit, as the damper unit may adventurously be parked beforethe step of rotating the damper unit. To facilitate parking of thedampen unit, a parking unit facilitating support of the damper unit onground may be provided.

In order to attach one of the at least two damper units to a towersection, the step of attaching the damper unit to one of the pluralityof tower sections may comprise a step of providing at least one throughhole in a wall of the tower section and fixing said damper to the towersection by a locking structure extending through the through hole. Theat least one through hole may be provided at the manufacturing site forthe turbine sections, whereby the through hole(s) may be provided aspart of the manufacturing process for the tower sections.

The locking structure may form part of the at least one damper unit,whereby it may be inserted into the at least one thought hole whenaligning the damper unit with the tower section.

Alternatively, the step of fixing the damper to the tower section by alocking structure extending through the through hole may be carried outby workers who are ready to fix a tower section to the tower sectionwhich is presently the uppermost tower section in the tower structure.As mentioned above, the workers may stand on a platform inside the towersection from which position they may insert the locking structure intothe through hole(s) to engage the damper unit.

Alternatively or additionally, the step of attaching one of the at leastone damper unit to an outside of one of the plurality of tower sectionsmay comprise a step of providing at least one shelf support on theoutside of the tower section and a step of fixing the damper unit to thetower section by engagement between the shelf support and a part of thedamper unit. The damper unit may be supported by the shelf support(s).In one embodiment, the shelf support(s) may comprise at least on recessinto which at least a part of the damper unit may be inserted to therebyfurther fix the damper unit to the tower section.

To facilitate damping of oscillations it may be an advantage, if thefirst and/or second damper unit is arranged in a predetermined position,such as in a vertical position. Thus, the step of attaching one of theat least two damper units to one of the plurality of tower sections maycomprise a step of providing at least one pivoting structure between thedamper unit and the tower section and a step of aligning the damper unitby reorienting the pivoting structure relative to the tower section. Thepivoting structure may form part of the damper unit or the tower sectionor may be a separate structure attachable to at least one of the damperunit and the tower structure. By reorienting the pivoting structure, itmay be assured that the damping unit is attached to the tower structurein the required orientation.

In one embodiment, the pivoting structure may be provided as at leastone foot attached to the damping unit. Consequently, the damping unitmay be aligned with tower sections of different size and/or differentouter geometry by reorienting the pivoting structure.

A damping frequency of at least one of damper units may be variable. Themethod may comprise a step of varying the damping frequency independency of the number of tower sections in the tower structure and/orin dependency of the natural frequency of the tower structure.Consequently, it may be possible to use the damping units at differentheight of the tower structure and thus reuse the damping unit(s) duringthe erection of the turbine tower. Because the damper may cater for thechanges in natural frequency of the tower during erection of the towerstructure.

According to a second aspect, the invention provides a damper unit foruse in a method according to the first aspect of the invention, thedamper unit comprising:

-   -   a damper unit structure adapted for attachment to the tower        structure,    -   a pendulum structure,    -   a suspension arrangement for suspending the pendulum structure        from the damper unit structure such that the pendulum structure        is allowed to displace from a neutral position for the pendulum        structure, the suspension arrangement comprising one or more        wires for suspending the pendulum structure,    -   a sensor adapted for measuring movements of the tower structure,        and    -   tuning means configured for adjusting the natural frequency of        the suspended pendulum structure in response to measured        movements of the tower structure.

It should be understood, that a skilled person would readily recognisethat any feature described in combination with the first aspect of theinvention could also be combined with the second aspect of theinvention, and vice versa.

The damper unit according to the second aspect of the invention is verysuitable for performing the method steps according to the first aspectof the invention. The remarks set forth above in relation to the methodare therefore equally applicable in relation to the damper unit.

According to a third aspect, the invention provides a hoisting systemfor use in a method according to the first aspect of the invention, thehoisting system comprising a tower section lifting sling and a damperunit lifting sling, wherein said damper unit lifting sling has a lengthexceeding a height of a tower section, whereby the hoisting system isattachable to a damper unit being attached to a tower section in thetower structure while being attached to an upper end of a subsequenttower section being arranged on top of said tower section.

It should be understood, that a skilled person would readily recognisethat any feature described in combination with the first and secondaspects of the invention could also be combined with the third aspect ofthe invention, and vice versa.

The hoisting system according to the third aspect of the invention isvery suitable for performing the method steps according to the firstaspect of the invention. The remarks set forth above in relation to themethod are therefore equally applicable in relation to the hoistingsystem.

According to a fourth aspect, the invention provides a method oferecting a wind turbine on a wind turbine site, the wind turbinecomprising a turbine tower and a nacelle, the method comprising thefollowing steps:

-   -   providing a plurality of tower sections being arrangeable upon        each other in a vertical orientation in a tower structure to        form the turbine tower,    -   providing at least one damper unit configured to dampen        oscillations of the turbine tower,    -   attaching a damper unit to one of the plurality of the tower        sections on an outside thereof, and subsequently arranging said        tower section in the tower structure, and    -   arranging the nacelle on top of the tower structure while said        damper unit is attached to a tower section in the tower        structure.

It should be understood, that a skilled person would readily recognisethat any feature described in combination with the first, second, andthird aspects of the invention could also be combined with the fourthaspect of the invention, and vice versa.

The damper unit according to the second aspect and the hoisting systemaccording to the third aspect of the invention are very suitable forperforming the method steps according to the fourth aspect of theinvention. The remarks set forth above in relation to the damper unitand the hoisting system are therefore equally applicable in relation tothe method. Furthermore, the method step of the first aspect are verysuitable for performing the method steps according to the fourth aspectof the invention. The remarks set forth above in relation to the methodaccording to the first aspect are therefore equally applicable inrelation to the method according to the fourth aspect.

A separate invention may be found in a method of erecting a wind turbinehaving a tower structure, the method comprising providing a plurality oftower sections, forming the tower structure by arranging the pluralityof tower sections upon each other in a vertical orientation in aprescribed tower lay-out, providing and installing the nacelle on top ofthe tower structure, providing at least one damper unit configured todampen oscillations of the tower structure during erection of the windturbine, attaching said at least one damper unit to an outside of atower section each, prior to arranging said tower section in the towerlay-out such that a damper unit is attached to at least one towersection during installation of the nacelle.

In an embodiment according to the separate invention the method isfurther defined in that at least two damper units are provided andwherein at least one of said at least two damper units are attached toan outside of a tower section at least during parts of the step offorming the tower structure.

In an embodiment according to the separate invention or the embodimentmentioned immediately above, the method further comprising providing andattaching a hoisting system to one of each damper unit and releasingsaid one damper unit from its tower section in the tower structure andlowering the damper unit to the ground.

In an embodiment according to the two embodiments mentioned immediatelyabove, the method comprising providing a hoisting system having a towersection lifting sling and a damper lifting sling, said damper liftingsling configured with a length such that it extends below next towersection to be installed, attaching the tower section lifting sling tothe next tower section to be installed, lifting the next tower sectionto be installed to a position adjacent the latest installed towersection, subsequently attaching the damper lifting sling to the damperunit on the latest installed tower section prior to arranging the nexttower section to be installed in the tower structure, releasing saiddamper unit on the latest installed tower section and lowering saiddamper unit to the ground.

In an embodiment according to the embodiment mentioned immediately abovethe method is further defined in that the damper unit released from thelatest installed tower section is reused for a subsequent next towersection to be installed.

The separate invention may be combined with any of the embodiments ofthe invention described previously.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be further described withreference to the drawings, in which:

FIGS. 1A and 1B illustrate attachment of a damper unit to a towersection,

FIG. 2 illustrates lifting of a tower section with a damper unitattached on the outside of thereof,

FIG. 3 illustrates lowering of the hoisting system,

FIGS. 4A and 4B illustrate lifting of a tower section, and subsequentlowering of a damper unit,

FIG. 5 illustrates positioning of a tower section, and subsequentlowering of a damper unit,

FIG. 6 illustrates lifting and positioning of a nacelle,

FIG. 7 illustrates lowering of a damper unit,

FIGS. 8A and 8B illustrate attachment of a damper unit to a towersection on ground,

FIG. 9 illustrates different embodiments of attachment of a damper unitto a tower section,

FIG. 10 illustrates a shelf support for a damper unit,

FIG. 11 illustrates pivoting structures for alignment of a damper unit,and

FIG. 12 illustrates rotation of a damper unit on ground.

DETAILED DESCRIPTION OF THE DRAWINGS

It should be understood that the detailed description and specificexamples, while indicating embodiments of the invention, are given byway of illustration only, since various changes and modifications withinthe spirit and scope of the invention will become apparent to thoseskilled in the art from this detailed description.

FIG. 1 illustrates steps of a method of erecting a wind turbine on awind turbine site. The wind turbine comprising a turbine tower 1 (seeFIG. 3) and a nacelle 2 (see FIG. 6). By the method a plurality of towersections 3 is provided on the site. The tower sections 3 are arrangeableupon each other in a vertical orientation in a tower structure to formthe turbine tower 1, where the “tower structure” is the part of theturbine tower 1 which has been erected at any time during the erectionof the wind turbine.

Furthermore, the method provides at least two damper units 4 configuredto dampen oscillations of the turbine tower 1. FIGS. 1A and 1Billustrate attachment of one of the at least two damper units 4 to oneof the plurality of the tower sections 3 on an outside thereof. In FIG.1A, a damper unit 4 is attached to the tower section 3 on an outsidethereof in a 12 o'clock position, a damper unit 4 in FIG. 1B is attachedto a tower section 3 on an outside thereof in a 9 o'clock position asviewed from the right hand side of FIG. 1B.

The damper unit 4 is attached to the tower section 3 by lowering(indicated by the arrow) the damper unit 4 towards the tower section 3by use of a hoisting system 5.

The tower section 3 while on the ground 6 is arranged on a supportstructure 7.

FIG. 2 illustrates lifting of a tower section 3 with a damper unit 4attached to an outside thereof. The tower section 3 is lifted by ahoisting system 5 and subsequently arranged on top of the towerstructure on the uppermost tower section in the tower structure.

FIG. 3 illustrates lowering of the hoisting system 5 after arranging atower section 3 in the tower structure comprising three tower sections 3forming the turbine tower 1. A damper unit 4 is attached to the towersection 3 which has being arranged before lowing the hoisting structure5.

FIGS. 4A and 4B illustrate lifting of a tower section 3′ (see FIG. 4A),and subsequent attachment of the hoisting system 5 to the damper unit 4(see FIG. 4B) on the tower section 3 already in the tower structure. Inthe illustrated embodiment, the hoisting system 5 comprises a towersection lifting sling 5A and a damper unit lifting sling 5B. The damperunit lifting sling 5B has a length exceeding the height of the liftedtower section 3′. Consequently, the step of attaching the hoistingsystem 5 to a damper unit 4 can be carried out while the hoisting system5 is attached to an upper end 3A of the lifted tower section 3′.Personnel who are assisting in the attachment of the lifted towersection 3′ to another tower section 3 already in the tower structure andto which tower section 3, the damper unit 4 is attached, are located ona platform (not shown) just below the top of the tower section 3 alreadyin the tower structure. The damper unit 4 is accessible to the personnelthat can reach the damper unit lifting sling 5B and attach it to thedamper unit 4 safely before the lifted tower section 3′ is aligned withthe tower structure.

FIG. 5 illustrates the steps following attachment of the damper unitlifting sling 5B in to the damper unit 4 in FIG. 4B after positioning ofa tower section 3′ with a damper unit 4′, and subsequent lowering of adamper unit 4. In the left part of FIG. 5, a tower section 3′ with asubsequent damper unit 4′ has just been arranged on top of another towersection 3 with a damper unit 4. The tower section lifting sling 5A isattached to the upper one of the tower sections 3′, whereas the damperunit lifting sling 5B is attached to the lower one of the damper units4.

In the centre of FIG. 5, the tower section lifting sling 5A has beenreleased from the upper tower section 3′. The hoisting system 5 islifted as indicated by the arrow to facilitate determination of tensionin the hoisting system 5. By sensing tension in the hoisting system 5,it may be possible to detect whether the damping unit 4 is attached tothe hoisting system 5 and fully supporting the weight of it.

In the right part of FIG. 5, the lower damper unit 4 has been releasedfrom the lower tower section 3 and is lifted to the ground by thehoisting system 5 while being attached to the damper unit lifting sling5B.

FIG. 6 illustrates lifting and positioning of a nacelle 2 on top of theuppermost tower section 3 by use of a hoisting system 5. The abovedescribed tower section lifting sling 5A is attached to nacelle 2 tothereby arrange the nacelle 2. In the same lifting step, the damper unitlifting sling 5B is attached to the damper unit 4 attached to theuppermost tower section 3. As indicated by the arrow A, the width of thepart of the nacelle 2 that extends outside the tower is smaller than thedistance between the tower 3 and the attachment point to the damper 4 ofthe damper unit lifting sling 5B. Therefore, the damper unit 4 can belifted without interference between the damper unit lifting sling 5B andthe nacelle 2 during lifting of the damper unit 4.

FIG. 7 illustrates lowering of a damper unit 4 after release of thedamper unit 4 from the uppermost tower section 3. As illustrated by thearrows, the hoisting system 5 is lifted to facilitate determination oftension in the hoisting system 5 to ensure that the weight of the damperunit 4 is fully supported by the hoisting system 5. Then the damper unit4 is released from the tower section 3 before it is lowered to theground.

FIGS. 8A and 8B illustrate attachment of a damper unit 4 to a towersection 3 in a horizontal orientation on the ground. The tower section 3comprises a plurality of through holes 8 in a wall of the tower sectionto allow fixing of the damper unit 4 to the tower section 3 by a lockingstructure 9 extending through the through holes 8. The locking structure9 forms part of the damper unit 4, whereby it can be inserted into thethought holes 8 when aligning the damper unit 4 with the tower section3.

FIG. 9 illustrates different embodiments of attachment of a damper unit4 to a tower section 3. The tower section 3 comprises a plurality ofthrough holes 8 in a wall of the tower section. In the left part of FIG.9, a locking structure 9 extends through each of the through holes 8 tothereby fix the damper unit 4 to the tower section 3. In the centre parof FIG. 9, a manual guide pin 10 is used to guide the damper unit 4 intoposition. Subsequently to the positioning, a locking member (not shown)may be inserted into the through hole 8. In the right part of FIG. 9, aguiding line 11 is used to guide the damper unit 4 into position. Aworker inside the tower section 3 may pull the guiding line 11 toposition the damper unit 4, and subsequently to the positioning, alocking member (not shown) may be inserted into the through hole 8.

The locking structure 9 is configured such that it can be released fromthe inside of the tower section 3. This makes is possible to release thedamper unit 4 upon attachment of a subsequent tower section 3′, see FIG.5, where the damper unit 4, and in particular any hoisting points orexternal connections to the tower, can no longer be reached by personnelfrom inside of the tower section 3.

FIG. 10 illustrates a shelf support 12 for a damper unit 4. The shelfsupport 12 is attached to a wall of the tower section 3. In theillustrated embodiment, the shelf support 12 extends from the inside tothe outside of the tower section 3, and comprises a reinforcementelement 13 on the inside. The damper unit (not shown) is attached to thetower section by engagement between the shelf support 12 and a part ofthe damper unit.

FIG. 11 illustrates pivoting structures 14 for alignment of a damperunit 4 with a tower section (not shown). The pivoting structures 14 arearranged between the damper unit 4 and the tower section. Asillustrated, the pivoting structures 14 are provided as feet attached tothe damping unit 4. The feet engage the outside surface of the towersection and pivots to automatically assume the best angle of contact.Consequently, the damping unit 4 may be aligned with tower sections ofdifferent size and/or different outer geometry.

FIG. 12 illustrates rotation of a damper unit 4 on ground 6. The damperunit 4 is located in a support structure 15. When the damper unit 4 hasto be rotated, the damper unit 4 is lifted together with the supportstructure 15 by the hoisting system 5. The rotational movement of thedamper unit 4 may be carried out to prepare the damper unit 4 forattachment to the tower section according to the orientation of saidtower section. For example attachment to a 3 o'clock, 9 o'clock or 12o'clock position on the tower section before lifting the tower sectionwith the damper unit 4 and arranging it in the tower structure.

The support structure 15 has three support parts 16, 16′, 16″ and twoattachment points 17′, 17″ for lifting slings 5′, 5″. To rotate thedamper unit 4 from the orientation in the left part of FIG. 12 to thaton the right part of FIG. 12, the lifting sling 5″ attached to thelifting point 17″ is tensioned until the damper unit 4 begin to rotateabout the corner support part 16′. Upon further tensioning of thelifting sling 5′ the damper unit 4 will rotate 90 degrees to assume theposition on the right part of FIG. 12. This provides for a safe and easyrotation of the damper unit 4.

1. A method of erecting a wind turbine on a wind turbine site, the windturbine comprising a turbine tower and a nacelle, the method comprisingthe following steps: providing a plurality of tower sections beingarrangeable upon each other in a vertical orientation in a towerstructure to form the turbine tower, providing at least one damper unitconfigured to dampen oscillations of the turbine tower, attaching adamper unit to one of the plurality of the tower sections on an outsidethereof, and subsequently arranging said tower section in the towerstructure, and arranging the nacelle on top of the tower structure whilesaid damper unit is attached to a tower section in the tower structure.2. The method of claim 1, wherein the nacelle is attached to a towersection lifting sling of a hoisting system to arrange the nacelle on topof the tower structure.
 3. The method of claim 2, wherein a towersection lifting sling of the hoisting system is attached to said damperunit before the nacelle is arranged on the tower section.
 4. The methodof claim 3, wherein the tower section lifting sling is released from thenacelle, and wherein said damper unit lifting sling is then used torelease said damper unit from the tower section.
 5. The method of claim4, wherein when said damper unit is lowered to the ground after it isreleased from the tower section.
 6. The method of claim 1, wherein thewidth of a part of the nacelle that extends outside the tower section issmaller than the distance between the tower and an attachment point tosaid damper unit of the damper unit lifting sling such that said damperunit can be lifted without interference between the damper unit liftingsling and the nacelle during lifting of said damper unit.
 7. The methodof claim 1, wherein the step of attaching said damper unit to one of theplurality of the tower sections on an outside thereof comprises a stepof providing at least one through hole in a wall of the tower sectionand fixing said damper unit to said tower section by a locking structureextending through the through hole.
 8. The method of claim 1, whereinthe step of attaching said damper unit to one of the plurality of thetower sections on an outside thereof comprises a step of providing atleast one shelf support on an outer surface of said tower section andfixing said damper unit to said tower section by engagement between theshelf support and a part of said damper unit.
 9. The method of claim 1,wherein the step of attaching said damper unit to one of the pluralityof the tower sections on an outside thereof comprises a step ofproviding at least one pivoting structure between said damper unit andsaid tower section and aligning said damper by reorienting the pivotingstructure relative to said tower section.
 10. The method according toclaim 1, wherein a damping frequency of at least one of the damper unitsis variable, and the method comprises a step of varying the dampingfrequency in dependency of the natural frequency of the tower structure.